Use

childProp:bind="key" is used to two-way bind a value in a ViewModel to
a value in the scope. If one value changes, the other value is updated.

The following two-way binds the <edit-plate> element’s plateName to the editing.licensePlate
value in the scope. This allows plateName to update if editing.licensePlate changes and
editing.licensePlate to update if plateName changes.

This demo can be expressed a bit easier with the references scope:

Initialization

When a binding is being initialized, the behavior of what the viewModel and scope properties
are set to depends on their initial values.

If the viewModel value is not undefined and the scope is undefined, scope will be set to the viewModel value.

If the viewModel value is undefined and the scope is not undefined, viewModel will be set to the scope value.

If both the viewModel and scope are not undefined, viewModel will be set to the scope value.